bcache: add read_super_common() to read major part of super block

Later patches will introduce feature set bits to on-disk super block and
increase super block version. Current code in read_super() which reads
common part of super block for version BCACHE_SB_VERSION_CDEV and version
BCACHE_SB_VERSION_CDEV_WITH_UUID will be shared with the new version.

Therefore this patch moves the reusable part into read_super_common(),
this preparation patch will make later patches more simplier and only
focus on new feature set bits.

Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>